1. Technical Field
The present invention relates to an image forming device and an image forming method performing a grayscale screen process.
2. Related Art
A line head provided to an image forming device has a number of light emitting sections corresponding to the printing width. By moving an image holding unit such as a photoconductor in a direction (sub-scanning direction) perpendicular to an arranging direction (main-scanning direction) of the light emitting sections, two-dimensional image formation is performed. In general, there are considerable difficulties in equalizing the light intensities of that number of light emitting sections. Further, even if the light intensities of the light emitting sections are even, unevenness in transmittance of the optical system for projecting the light beams from the respective light emitting sections on the image holding unit results in uneven light intensity distribution on the image holding unit. Such unevenness in light intensity distribution causes vertical stripes or belts on an image, which degrade the image quality. In order for solving this problem, it is generally conducted to make the light intensity distribution on the image holding unit even by correcting the light intensity of every light emitting section.
However, there are some cases in which it is still difficult to obtain an even image only by equalizing the light intensity distribution on the image holding unit. A fiber lens array (e.g., SELFOC™ lens array, product of Nippon Sheet Glass Co., Ltd.), which is often used for focusing an image of the light emitting sections on the image holding unit, causes unevenness in shape and size of the image on the image holding unit depending on variation in characteristics and an arrangement of the fibers used therein, and may cause defects in the image derived from the unevenness. In order for avoiding the image defects caused by the variation in the focusing performance, JP-A-8-142406 discloses a method of correcting variation in image quality by controlling the light intensity in accordance with breadth of the light intensity distribution of the beam spot focused thereon.
Meanwhile, in digital printing of a grayscale image, the image is often expressed using area-modulation. Namely, the contrasting density of an image is expressed by the size of halftone dots or the thickness of lines. However, if the focusing conditions are different in every pixel forming the screen, variation in the contrasting density is caused for every pixel. Further, the degree of variation in the contrasting density may vary depending on the screen process of the image to be printed. In order for solving such a problem, JP-A-2004-148652 shows a method of calculating a light intensity correction value for every light emitting section (pixel) in accordance with given information regarding the angle of a screen.
The related art as described above can exert the effect of eliminating the variation in contrasting density of an image with specific kind of screens, but there were some cases in which the effect was not exerted with different kind of screens. FIGS. 7A through 7C are explanatory diagrams each showing an example of the structure of a screen. FIGS. 7A and 7B show examples of the screens having the same angles but different pitches (number of lines). As shown in the drawings, in the cases with the same screen angles and different screen pitches (number of lines), the same correction values exert different effects. Further, FIG. 7C shows an example of a screen having different structure from those shown in FIGS. 7A and 7B. As shown in the drawings, in the cases having different screen structures, it is no longer possible to simply determine the correction values in accordance only with the screen angles.
Further, images to be input to the image forming device are not limited to natural pictures (photographs), but include various types of images, such as line drawings, graphics, or characters. A type of a screen to be used is selected in accordance with the type of the image as described above. For example, screens with fine pitch as shown in FIG. 7A are used for graphs, drafts, or line drawings, which require high resolution. And, for natural pictures such as photographs, which give greater importance to gradiation than resolution, screens with rather rough pitch as shown in FIG. 7B are used. Since this kind of selection is performed even in the same page, even among pixels lighting at the same turn, different screens may be used therefor according to location. In these cases, the coping method of using static screen information throughout the same page as shown in JP-A-2004-148652 does not work.